This invention relates to a protective relay used in a system for protecting one or more zones of an A.C. high voltage transmission line, and more particularly, to a circuit for a protective relay to develop a plurality of characteristic angles.
Protective relays are typically situated at a particular location along an A.C. high voltage transmission line for the protection of portions or zones along the transmission line. Protective relays frequently employ a phase comparator method of measurement to provide protection for zones of the transmission line. One such protective relay is described in U.S. Pat. No. 4,161,011 of S. B. Wilkinson. Another such protective relay is described in U.S. patent application of L. P. Cavero, Ser. No. 309,549 filed Oct. 7, 1981. This patent and the patent application are both assigned to the same assignee of the present invention and are herein incorporated by reference.
The protective relay typically has parameters related to the protected zones and to the transmission line. For example, the protective relays have a "forward reach" parameter, a "reverse reach" parameter, and a characteristic angle parameter. The forward reach is the distance in a forward direction, relative to the location of the protective relay, for which the relay is desired to detect and respond to faults along the transmission line. The reverse reach is similar to the forward reach but in the opposite direction along the transmission line. The forward and reverse reach parameters are typically selected to conform to the distances or zones of the transmission line. The reach of a protective relay related to the phases A and B of the transmission line may typically be determined by the expression Z.sub.L1AB =V.sub.AB /(I.sub.A -I.sub.B), where the subscripts A and B are related to the phases A and B of the transmission line. The quantity Z.sub.L1AB is the positive sequence impedance of the transmission line related to phases A and B between the location of the protective relay and the boundary of the zone along the transmission line desired to be protected. The protective relay frequently in determining a fault condition related to phases A and B, through appropriate coupling devices, compares the potential (V.sub.AB) between the phases A and B with the vectorial difference (I.sub.A -I.sub.B) of the phases A and B currents. The quantity V.sub.AB coupled to the protective relay is typically termed the restraint signal. The signals representative of the I.sub.A and I.sub.B are typically coupled to the protective relay and combined with a Z.sub.R1 quantity, which is the positive sequence impedance of the protective relay itself, to form the quantity (I.sub.A -I.sub.B) Z.sub.R1, which, in turn, is commonly termed one of the operate signals of the relay.
The characteristic angle parameter of the protective relay is typically selected to correspond to the characteristic angle parameter of the protected zone relative to the impedance angle of the transmission line.
For example, for a transmission line of a 60 Hz system the impedance angle may have a range of 45.degree.-85.degree.. The impedance angle is dependent upon the resistive, capacitive and inductive parameters of the transmission line. The impedance angle, is the angle by which the current flowing in the transmission lags the voltage drop across the transmission line. The characteristic angle of the relay is typically selected to be approximately equal to the impedance angle of the transmission line. For example, if the impedance angle of the transmission line is 82.degree., the characteristic angle of the protective relay may be set to 80.degree.. The term "angle of maximum reach" of the protective relay is commonly used in an interchangeable manner with the characteristic angle. It is desirable that the characteristic angle parameter of the protective relay be easily adjusted or adapted so as to allow the protective relay to match the impedance angle of the protected transmission line as closely as practical in order to provide optimum protection.
The characteristic angle of the protective relay may typically be developed from a signal IZ coupled from the transmission line to the protective relay. The characteristic angle may be selected by adjusting the transfer impedance angle of a coupling device, such as a transactor, coupled to one of the phases of the phases of the A.C. transmission line so as to sense the current (I) flowing within the coupled phase. The transactor develops the signal IZ, where Z is the transfer impedance of the transactor having a transfer impedance angle .theta.. If the transactor is coupled to phase A of the transmission line the signal IZ may be expressed as I.sub.A Z.sub.R1 which is one component of the operate signal (I.sub.A -I.sub.B) Z.sub.R1 and whose magnitude also affects the reach of the protective relay. It is desired that the magnitude of the developed signal IZ of the transactor remain a substantially constant quantity in order that the reach quantity of the relay may remain a substantially constant quantity.
The phase angle between the signal coupled into the transactor and the signal output IZ, controlled by the transfer impedance Z having a transfer impedance angle .theta. of the transactor, is typically selected to correspond to the characteristic angle of the protective relay. The development of a characteristic angle employing a transactor has limitations with regard to its ability for changing or varying the characteristic angle without effecting the reach of the relay. For example, as the transfer impedance Z of the transactor is varied not only does the desired phase angle correspondingly vary, but also the magnitude of the output signal IZ from the transactor is varied, which causes the reach of the relay to vary also.
Variable gain operational amplifiers are commonly employed to compensate for voltage changes caused by changes in the transactor that would otherwise effect the reach of the relay. The variable gain operational amplifier may typically be coupled to the output of the transactor by a plurality of loading resistors. At least two resistors must be varied for each desired characteristic angle. If only two or possibly three different characteristic angles are desired to be selectable for a protective relay, this type of compensation serves its desired function. However, if a multiplicity of characteristic angles are desired, this type of compensation typically necessitates a large number of resistors, which, in turn have a disadvantage with regard to equipment mounting, more particularly, with regard to circuit board mounting. It is desirable that means be provided for adjusting or preselecting a plurality of characteristic angles that utilizes a substantially reduced number of resistors for its implementation.
The characteristic angle of the protective relay may be developed by means other than a transactor. For example, the protective relay may develop two types of signals: (1) an operate signal V.sub.OP and (2) a polarizing signal V.sub.POL, by an appropriate phase shifting network. The previously mentioned U.S. patent application, Ser. No. 309,549, describes such V.sub.OP and V.sub.POL signals. The characteristic angle may be developed by phase shifting the V.sub.POL signal with respect to the V.sub.OP signal. This method also results in the reach of the relay being a function of the characteristic angle. That is, the reach of the relay is typically proportional to the reciprocal of the cosine of the phase shift angle. As the phase shift is altered, the reach of the relay is also altered. This method of phase shifting distorts the desired shape of the desired relay characteristic. For example, the V.sub.OP and V.sub.POL are typically applied to a coincidence circuit, that is, an "AND" circuit, whose output is routed to a timer circuit. The timer is typically set to develop an output signal when the coincidence angle between V.sub.OP and V.sub.POL equals 90.degree. which produces a circular relay characteristic. The variation of this coincidence angle can produce other shapes for the relay characteristic which are desirable for some applications. For example, if the coincidence angle is selected to correspond to a value of greater than 90.degree., a "lens" shape for the characteristic of the relay is developed. Further, if the coincidence angle is selected to correspond to a value of less than 90.degree., a "tomato" shape for the characteristic of the relay is developed. This method of phase shifting the polarizing signal distorts the desired lens or tomato shapes of the relay characteristic.
Accordingly, objects of the present invention are, (1) providing means for accurately selecting a plurality of characteristic angles for a protective relay using a reduced number of resistors, and (2) providing a protective relay with a variable characteristic angle while maintaining (a) a desired shape for the relay characteristic and, (b) a substantially constant reach function.
These and other objects of the invention will become apparent to those skilled in the art upon consideration of the following description of the invention.